Communities that have thrived for centuries in Nepal's rugged mountain environments are facing rapid population declines caused by the outmigration of youths, both males and females in nearly equal numbers, who are sent by parents to distant boarding schools and monasteries for secular and religious education. This paper documents the magnitude of outmigration, migration destinations, migration's impact on the age–sex composition of sending communities, the effect of migration on fertility, and projected trends of population decline and aging. The authors conclude by discussing potential long-term threats to the viability of ethnically Tibetan communities in the Himalayan highlands, including outmigration's effect on agricultural production, the family-based care system for the elderly, socioeconomic inequalities, and human capital.